U.S. Pat. No. 8,585,543 B1 describes a system and a method for controlling the speed of a working machine, which comprises a continuous transmission, namely a so-termed CVT (continuously variable transmission) which is functionally connected to a drive source. Associated with the CVT are a number of selectable virtual gear steps. The working machine comprises a machine operator's input device which can produce a machine operator's input signal that specifies a desired setting of the basic speed of the working machine. The machine operator's input signal can be converted to a CVT rotational speed command and to a drive source rotational speed command, which are sent to the CVT transmission and the drive source respectively. The CVT transmission adjusts the machine's speed in accordance with the CVT rotational speed command while the drive source adjusts its rotational speed in accordance with the drive source rotational speed command, so that the operation of the drive source is essentially independent of the operation of the CVT transmission.
With appropriate design of such a CVT transmission, in the area of the CVT transmission a so-termed infinite transmission ratio can be obtained, in order to be able to operate a drive motor of a vehicle drive-train in the form of an internal combustion engine without having to decouple it to the usual extent from the drive output of the vehicle drive-train by means of a shifting element or the like. This means that if an infinite transmission ratio is set in the area of a CVT transmission, the drive output rotational speed can be regulated to 0 revolutions/minute while at the same time the drive motor is still rotating. If in the area of a CVT transmission this so-termed infinite transmission ratio is set, the traction force needed for a working machine can be produced with a rotational speed and a torque of the drive motor which are lower than the maximum motor rotational speed and the maximum torque that can be produced by the drive motor.
Following the general tendency to keep the fuel consumption of working machines as low as possible, it is therefore logical when obtaining very high transmission ratios close to the infinite transmission ratio, to reduce the rotational speed of the drive motor without thereby compromising the working performance of the working machine. In general, if the rotational speed level of the drive motor is reduced at the same time that a high transmission ratio is set in the area of a CVT transmission in order to reduce the fuel consumption, then during unfavorable variations of the operating condition of a working machine, during which a driver only calls for a low vehicle speed but by virtue of a working hydraulic system the working machine is trying to generate a lot of power, in some circumstances the drive motor cannot produce sufficient drive power for this. In order by means of the drive motor to make sufficient drive power available for this, the drive motor must be operated at a correspondingly high rotational speed, but this cannot be done to the extent desired by the procedures known until now.